The invention relates to improvements in traveling web coating mechanisms and more particularly to an improved air knife coater of the type used in paper web coating.
While the principles of the invention may be employed in coating various materials, the features are particularly well suited for use in coating a traveling paper web, and the preferred embodiment will be described in this environment. In air knife coating, a layer of coating is first applied to a traveling web, and the web is supported for stability on a backup roll with a curtain of relatively high velocity air directed against the oncoming web to smooth the coating surface and in some instances to doctor off excess coating. The velocity of air emitted from the air knife which is necessary for a satisfactory doctoring process and the speed of the oncoming web will tend to create a mist of particles of coating. The air knife coater includes a head with a pair of lips extending toward the smoothing zone where the curtain of air is emitted, and the entrained particles of mist which are formed in the zone tend to deposit on the lip of the coater, particularly on the downstream side of the smoothing zone. These particles will build up and require stopping the machine for cleaning. If they are not cleaned off, they will build up to a degree where they will fall off and cause surface defects in the moist coating, which condition cannot be condoned. The mist of airborne particles will increase as the speed of the coater increases and particularly with certain types of coating. In addition to depositing on the lip of the coater on the offrunning side, the particles of mist tend to become partially dried in the air and deposit on the coated web to cause imperfections. The mist, to some degree, will also coat surrounding machine parts, and is annoying and disadvantageous to personnel in other operations in the surrounding area. Attempts have heretofore been made to reduce the mist, but these devices have encountered difficulties and have not proven entirely successful.
It is accordingly an object of the present invention to provide an air knife coater embodying a method and arrangement for reducing and substantially eliminating the deposit of entrained coating mist particles which occurs on the downstream lip of the air knife coater.
A further object of the invention is to provide an air knife coater wherein there is not a continual deposit and build-up of coating particles on the air knife which require the nuisance and expense of stopping the machine.
A still further object of the invention is to provide an air knife coater wherein fine debris and coating dust that passes from the smoothing zone and is generated by impingement of the air jet on the coating is removed from the critical zones of operation and prevented from depositing on the machine.
A feature of the invention in the preferred embodiment employs a saveall below the smoothing zone of an air knife coater having a wall that projects toward the smoothing zone and is spaced from the air knife coater so that a flow of air can pass between the coating head and the wall. The structure also employs a baffle on the offrunning side of the smoothing zone, which baffle extends somewhat parallel to the upper, or downstream lip, of the coating head so that a flow of air continually passes along the downstream lip toward exhaust outlet passages carrying away entrained mist particles at a velocity preventing their deposition on the air knife lip or passing web.
Other objects, features and advantages of the invention will become more apparent with the teachings of the principles thereof in connection with the disclosure of the preferred embodiments thereof, as will equivalent structures which are fully intended to be covered herein, in the specification, claims and drawings, in which: